We intend to study factors affecting changes in transmission through a reflex circuit in the mammalian central nervous system. The reflex chosen for investigation is the plantar cushion (PC) reflex, a cutaneous, spinal reflex in the cat. The neural circuit of the PC reflex appears to consist of not more than two serially connected populations of interneurons, interposed between the cutaneous afferents and the motoneurons to the intrinsic plantar muscles. Further neurophysiological analysis, employing extra- and intracellular recording of single spinal units, will be undertaken to clarify the circuit components of the PC reflex. In addition, factors that affect transmission through the reflex will be investigated. Repeated stimulation at 2-10 Hz typically leads to enhanced transmission (sensitization) followed by waning (habituation) of the reflex. Changes in transmission during repeated stimulation have been found to occur in the region of the first-order interneurons. Whether these changes are pre- or postsynaptic, and whether they involve intrinsic changes in the reflex neurons, or modulation via pre- or postsynaptic input from other neurons, will be investigated. Also an attempt will be made to condition the PC reflex. If the PC reflex can be classically conditioned, it will be used to study the neural mechanisms related to changes in transmission occurring during conditioning.